Caging and excited state emission of ICN trapped in cryogenic matrices: experiment and theory

We discuss the cage induced stabilisation of fragments in excited electronic states following the UV-dissociation of ICN in cryogenic matrices. Emission spectra recorded upon (A) over tilde-band excitation of ICN in solid neon, argon and krypton exhibit a long progression of broad bands due to a weakly bound electronically excited state, presumably one of the low-lying triplet states (3)Pi(1) or (3)Pi(2) of ICN. A lifetime analysis favours the (3)Pi(2) state. Molecular dynamics with quantum transitions (MDQT) simulations were conducted on six coupled electronic potential energy surfaces in a matrix of 498 argon atoms. Although a complete potential energy surface for the (3)Pi(2) state is not available, it is known to be very similar to the (3)Pi(1) one. Therefore only the 6 available [(3)Pi(1) (A', A), (3)Pi(0+), (1)Pi(1) (A', A), X(1)Sigma(+)] ab initio electronic potential energy surfaces were considered. The results predict a 2% probability of stabilisation in the shallow minimum of the triplet excited state. The molecule adopts a linear ICN configuration with a mean value of the I-CN distance far away from the absorption Franck-Condon region. The simulations also deliver insight into the mechanism of cage-induced population trapping in excited state surfaces, which is not accessible in the gas phase.